This invention relates generally to the field of electric motors and particularly to the field of controlling electronically commutated motors.
With the advent of reliable, low cost power electronics, an application that once used a brush-type, DC (direct current) motor, is now likely to employ a smaller, safer, cleaner running, longer-lived, and less expensive electronically commutated motor (henceforth, ECM).
The ECM is a permanent magnet electrical machine with the magnet (or magnets) on the rotor and multiple, spatially distributed phase windings on the stator. Current in the windings interacts with the permanent magnetic field to produce the machine""s torque. To maintain a constant torque as the rotor turns, the current distribution in the stator is continually adjusted to maintain a constant spatial relationship with the rotor""s magnetic field. The adjustment in current distribution is accomplished by switching (xe2x80x9ccommutatingxe2x80x9d) current among the various stator winding phases. In a brush-type DC motor, where the magnets are stationary and the windings rotate, commutation occurs mechanically through the interaction of brushes with a commutator ring. In an ECM, as the name implies, commutation is effected electronically by controlling the conduction states of a multiplicity of electronic power devices electrically coupling the various stator phase windings to a power bus. A subsystem comprising the power devices and any apparatus required to realize a set of power device conduction states appropriate to a respective set of device control signals is called a xe2x80x9cpower amplifierxe2x80x9d; a subsystem comprising any apparatus used to generate the set of device control signals so as to cause the ECM phase currents to track a set of current reference signals is called a xe2x80x9ccurrent controllerxe2x80x9d; a subsystem comprising any apparatus used to generate the set of current reference signals so as to cause the motor torque to track a torque reference signal is called a xe2x80x9ctorque controllerxe2x80x9d; a subsystem comprising a power amplifier, a current controller and a torque controller is called a xe2x80x9cmotor drivexe2x80x9d.
ECMs are generally divided into two broad classes, distinguished by the way the stator windings are spatially distributed, and named according to the shape of the back EMF (electromotive force) waveforms generated. A motor with a concentrated winding generates a trapezoidally shaped back EMF waveform and is referred to as a xe2x80x9ctrapezoidal ECMxe2x80x9d a motor with a sinusoidally distributed winding is a xe2x80x9csinusoidal ECM.xe2x80x9d Conventionally, motor drives used with either class of motor generate phase current waveforms that have a similar shape to and are aligned/with the back EMF waveforms of the respective phases. This current shaping strategy constrains the torque to remain unidirectional. While both motor classes may use the same power amplifier and current controller designs, torque controller designs generally differ between the two.
Sinusoidal ECM systems generally employ an explicit rotor angle sensor such as, for example, an optical shaft encoder or a resolver. The torque controller acquires the rotor angle sensor output and generates current reference signals that are sinusoidal functions of the measured angle.
In contrast, trapezoidal ECM systems often employ an implicit rotor angle sensing technique. Because the phase current is intermittently conducted, trapezoidal systems can achieve an angle measurement by sensing the back EMF at the terminals of non-conducting phases and generating a signal whenever a prescribed voltage threshold is crossed. These threshold crossing signals initiate step changes in the current reference signals for the appropriate phases.
In general, compared to a sinusoidal ECM system, a comparably sized trapezoidal system has the advantage of being less expensive owing to the absence of an explicit angle sensor, the relative simplicity of the torque control algorithm, and the relatively simple construction of the concentrated winding motor. A trapezoidal ECM system has the disadvantage, however, of producing significantly higher levels of undesirable torque ripple.
Those applications where cost is the primary concern and torque ripple is a secondary consideration have been traditionally well served by trapezoidal systems. In contrast, those applications requiring lower torque ripple have been traditionally satisfied by more expensive solutions using sinusoidal ECMs. Applications where the torque ripple of a conventional trapezoidal ECM system would be unacceptable, yet the cost of a conventional sinusoidal ECM system would be prohibitive are not well served by either solution. One such application, for example, would be a variable speed blower for high efficiency heating, ventilation, and air conditioning in commercial and domestic buildings. An opportunity exists, therefore, to address these orphan, low ripple, low cost applications.
By combining conventional trapezoidal motor, power amplifier, and current controller designs with a novel torque controller design, embodiments of the present invention provide a system with a torque ripple performance approaching that of a sinusoidal system at a cost approaching that of a trapezoidal system.
A method and apparatus are disclosed for shaping current in an electronically commutated motor (ECM), the method comprising: acquiring a set of terminal voltage measurements; estimating an instantaneous rotor angle from the terminal voltage measurements; generating a set of phase current commands from the instantaneous rotor angle and from a torque reference signal, each of the phase current commands comprising at least one ripple compensation command segment and at least one zero current command segment; and converting the set of phase current commands to a set of current reference signals.